Extended bandwidth, i.e., relatively unchanging performance characteristics with variation in excitation frequency, is often desirable in antennas. Certain antenna configurations have inherently good bandwidth, such as helical antennas.
Helical antennas typically include a cylindrical, electrically insulative substrate upon which a helical, electrically conductive member is disposed. As will be more fully explained below, where the circumference of the helical member is made equal to the wavelength of the antenna excitation frequency, one achieves the so-called "axial" or "beam" mode of radiation, wherein a lobe of desired narrow transverse expanse radiates axially of the helical member and persists, with generally circular polarization, over a relatively wide frequency range.
As a general rule, reported in the literature, from a lower excitation about three-quarters of the helical member circumference in wavelengths to an upper excitation wavelength of about four-thirds of the helical member circumference in wavelength, one achieves such axial mode radiation pattern and radiation persistency characteristic.
It may be said that the helical antenna is inherently impedance-matched over such radiation persistency range, since radiation persistency or uniformity implies that the antenna is impedance-matched. However, as excitation frequency extends beyond such lower and upper wavelength limits, the helical antenna exhibits problematic performance, which likewise may be attributed to it being impedance-mismatched.
Certain art-recognized parameters of helical antennas are shown in FIGS. 1 through 4. Referring thereto, a known helical antenna 10 includes a generally cylindrical and electrically insulative substrate 12 upon which is disposed a helical, electrically conductive member 14, having a lower starting end 16 and an upper end 18, such ends being generally circumferentially coincident. A ground plane member 20 of electrically conductive material is adjacent helical member end 16. The circumference C of helical member 14, of course, is equal to the product of pi (3.1416) times the diameter D of member 14. A spacing S exists between ends of each individual turn of member 14. As is shown in the geometric diagram of FIG. 4, the length L of each turn, in rectilinear dimension, in unwound condition, is the hypotenuse of a triangle having mutually orthogonal sides C and S, giving rise to definition of a pitch angle A. The foregoing axial mode persistency and broadband characteristic applies where C is generally equal to the excitation wavelength and the pitch angle is relatively small, for example, from about ten to sixteen degrees.
While the art generally recognizes a variety of impedance matching elements, typically in the form of lumped reactance elements such as coils and capacitors, complexity and cost attends these impedance matching schemes and they are generally operative over only a limited extended frequency range.
In addition to the need for improved and simplified schemes for impedance matching to achieve enchanced bandwidth, the art looks in various instances to performance uniformity among separate antennas performing interrelated functions, such as in transmit-receive systems. For example, well-known electronic article surveillance (EAS) systems have mass produced antennas respectively for radiating energy into a controlled or surveillance zone to impinge upon tags affixed to articles and for receiving energy returned from tags for alarm output indication under certain conditions. Since uniformity in transmission and reception is desired, antenna characteristic sameness with change in frequency is of significance.